Pressure sensor with temperature compensated optical fiber

ABSTRACT

The present invention relates to a temperature-compensated optical fiber pressure detector for detecting pressure variations in at least one medium in relation to a reference medium such as the atmosphere or a medium where a back pressure prevails. The detector essentially comprises a deformable element ( 2 ) such as a bellows ( 5 ) exposed on one side to the reference medium and on the opposite side to the pressure to be measured, an optical fiber portion (F 1 ) including at least one optical grating such as a Bragg grating (B 1 ), which is connected on one side to deformable element ( 2 ) and on the opposite side to a fixed point. The optical fiber portion is subjected to a prestress by a device ( 6, 15, 20, 21 ) and its elongation varies with the displacements of the deformable element. The device applies the prestress to optical fiber portion (F 1 ) between deformable element ( 2 ) and another fixed element ( 15 ) isolated from the medium by a rigid housing ( 1 ). An optical system ( 25 ) detects the deformations undergone by said optical grating in response to the pressure variations undergone by the deformable element. Another part (F 2 ) of the optical fiber that is not subjected to stresses (or another non-stressed fiber portion connected to the first one) preferably comprises another similar optical grating (B 2 ) also allowing measurement of the temperature variations. By duplicating the deformable elements and possibly the stress applying device ( 3 ), differential pressure variations can also be measured. Such a detector can notably be applied to pressure and temperature measurements in wells where difficult conditions prevail.

FIELD OF THE INVENTION

The present invention relates to a temperature-compensated optical fiberpressure detector.

The detector according to the invention finds applications in manyspheres measurement, control, detection-alarm, monitoring, whereexcellent immunity to the outside environment is sought. It isparticularly well-suited notably for measurement of the pressureprevailing in oil production wells. Although it can be used for pressuremeasurements in general, the detector is more particularly intended forvery high precision measurement of the pressure variations within afluid.

BACKGROUND OF THE INVENTION

Optical fiber detectors afford many specific advantages: limited spacerequirement, reduced mass, large passband and low attenuation, immunityto electromagnetic interferences, good resistance to the effects ofionizing radiations, possibility of multiplexed reading of the signalsproduced by various detectors and considerable measuring point offset,etc.

The prior art in this sphere can be illustrated notably by the followingpatents: U.S. Pat. No. 4,932,262; U.S. Pat. No. 5,317,929; U.S. Pat. No.5,600,070; WO-99/13,307 (U.S. Pat. No. 6,016,702); WO-00/00,799;WO-01/14,843 or EP-1,008,840 A1.

The pressure detectors comprise for example one or more deformableelements (diaphragm, bellows, etc.) one surface of which is subjected tothe pressure prevailing in a medium, the other surface being subjectedto a reference pressure. The deformations or displacements of theseelements under the effect of the pressure variations in the medium aretranslated into variations in the length of an optical fiber portionwith Bragg gratings that connects the mobile element to a fixed housing.A detector of the same type is often associated with these pressuredetectors, on an optical fiber portion that is not subjected to a stresswhere the grating deformations are only due to temperature variations.Standard type Bragg gratings can for example be used, whose meanspectral width is of the order of 200 pm or, for higher precision, phasejump type Bragg gratings whose spectral width is reduced to somepicometers (pm), as described for example in patent applicationWO-9,959,009.

SUMMARY OF THE INVENTION

The optical fiber pressure detector according to the invention allows todetect pressure variations in a medium in relation to a reference medium(the atmospheric pressure for example or a back pressure). It comprisesat least one deformable element (a membrane for example, or preferably abellows whose length increases when the pressure of the medium rises)exposed on one side to the reference medium and, on the opposite side,to a pressure to be measured, and at least one optical fiber at least aportion of which is prestressed. It is therefore connected on one sideto the deformable element and, on the opposite side, to a referencepoint so that its prestress varies with the displacements of thedeformable element. This optical fiber portion comprises at least oneoptical grating and an optical system for detecting the deformationsundergone by each optical grating as a result of the pressure variationsundergone by the deformable element.

The detector is first distinguished in that it comprises a deviceresting on a second rigid element that constitutes the reference point,for applying an adjustable prestress to the or to each fiber portion(this second rigid element being preferably isolated from the medium bythe outer housing). By means of this device, the desired prestress to beapplied to the fiber can be readily adjusted locally.

According to an embodiment, the device comprises prestress means restingon the second rigid element (such as one or more rigid parts that can bemoved away from this second element) and retention means associated withtwo opposite ends of each fiber portion which cooperate with theprestress means so as to apply an adjustable prestress to this fiberportion.

According to an embodiment, the device comprises means for giving eachoptical fiber portion the shape of an open loop formed between thedeformable element and the retention means associated with the prestressmeans.

It can therefore comprise a semi-circular path (consisting for exampleof the groove of a pulley portion whose axle is fastened to thedeformable element) associated with the deformable element, saidprestress means resting against a wall of the second rigid element.

According to an embodiment, the device comprises means for giving eachoptical fiber portion the shape of a rectilinear fiber element formedbetween the deformable element and the second rigid element, theprestress means resting against this second rigid element.

Different variants can be applied to the previous two embodiments. Themeans for prestressing the fiber portion can for example comprise a stoppiece resting against the second rigid element, and spacing means (byscrewing for example) for moving the stop piece away from said secondrigid element, the optical fiber portion retention means beingassociated with the stop piece. The prestress means can also comprisemeans for locking the stop piece in relation to the second rigidelement.

The retention means can also comprise two mechanical latching elementssuited to keep each fiber portion locked respectively at its oppositeends, one being fastened to the deformable element and the other to apart that can be translated in relation to the second rigid element, andmeans for adjusting to a predetermined value the prestress applied tothe fiber portion.

The retention means can comprise at least one locking part embedded inthe stop piece which cooperates with at least one local oversize of theoptical fiber portion or at least one part set on the fiber portion, ormechanical latching elements suited to keep each fiber portion lockedrespectively at its opposite ends, one fastened to the deformableelement and the other to the stop piece, or at least one locking partembedded in the stop piece pierced with a calibrated hole, each fiberportion being associated with each locking part by any known means(notably by sticking).

According to a preferred embodiment, the deformable element comprises abase secured to the body and a movable part (such as a bellows withcorrugations of various possible shapes: symmetrical or asymmetrical,helical, with corrugation amplitudes of the inner and outer helices thatcan be equal or different) secured to the base, and the second rigidelement comprises a rigid tube secured to the base and inside the body.Protection means are preferably added to limit deformation of thedeformable element.

The detector can comprise an outer sheath made of a thermally insulatingmaterial.

According to a possible embodiment, the body is made of a material oflow thermal conductivity and the elements of the detector inside thebody are made of materials whose thermal conductivities are selected tobest minimize the formation of thermal gradients.

According to a preferred embodiment, the detector comprises at least asecond optical grating on another fiber portion that is not subjected toa prestress, so as to compensate for the stress variations due totemperature variations.

In order to best correct the effects of the temperature on the pressuremeasurements, the detector can comprise a plurality of optical gratingsdistributed over at least one fiber portion that is not subjected to aprestress, so as to detect thermal gradients inside the body, theoptical system being suited to combine the measurements of the variousgratings.

In order to best limit the measurement biases due to imperfections inthe latching system, it is in any case preferable that the base lengthof the prestressed optical fiber portion is sufficiently great. Thethermal expansion coefficients of the constituent elements of thedetector are also preferably selected to best minimize the lengthvariations of the prestressed fiber portion under the effect of thetemperature variations.

The detector according to the invention can be readily adapted tomeasure relative or differential pressure variations while keeping thesame adjustability performance as regards the prestress applied to theor to each fiber portion.

According to a first embodiment suitable for measuring the absolutevalue of a difference between two pressures, the detector comprises forexample two deformable elements exposed on one side to the referencemedium and, on the opposite side, respectively to two pressures to bemeasured, at least one optical fiber portion subjected to a prestress,which is connected on one side to one of the deformable elements and, onthe opposite side, to a rigid part of the other deformable elementforming said second rigid element, and whose stress varies with thedisplacements of the two deformable elements, this prestressed opticalfiber portion comprising at least one optical grating.

According to a second embodiment suitable for measuring the amplitudeand the sign of the relative variation, the detector comprises forexample two deformable elements exposed on one side to the referencemedium and, on the opposite side, respectively to two pressures to bemeasured, at least two optical fiber portions each subjected to a priorprestress, which are connected on one side respectively to the twodeformable elements and, on the opposite side, to at least one referencepoint that is fixed, and whose respective stresses vary separately withthe displacements of the two deformable elements, each one of theseprestressed optical fiber portions comprising at least one opticalgrating.

The detector can of course comprise on each optical fiber a plurality ofgratings so as to increase its resolution.

Optical fibers provided with standard or phase jump type Bragg gratingsare for example used.

The pressure detector according to the invention is notably advantageousin that:

-   the desired fiber prestress can be readily adjusted locally by    moving away a locking element,-   with some of the embodiments described, it is possible, without    reflection-generating parallel connection or discontinuity, to    arrange several detectors in series at a distance from one another,-   the mechanical elements forming the fixed reference point are    preferably located within a housing and insulated from the outside    pressure (not subjected to deformations).

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the method according to the inventionwill be clear from reading the description hereafter of a non limitativeembodiment example, with reference to the accompanying figures wherein:

FIG. 1 diagrammatically shows a first embodiment of the pressuredetector in axial section, comprising a prestressed optical fiber loop,

FIG. 2 is a radial cross-sectional view of the pulley round which theoptical fiber runs,

FIG. 3 is a cross-sectional view of the baseplate of the inner tube,

FIG. 4 is a cross-sectional view of the stop plate,

FIG. 5 diagrammatically shows a first embodiment variant of thefiber-loop pressure detector, suitable for differential pressuremeasurement,

FIG. 6 diagrammatically shows an embodiment variant of the fiber-looppressure detector, suitable for differential pressure measurement,

FIG. 7 diagrammatically shows a second embodiment of the pressuredetector in axial section, comprising a prestressed linear fiberportion,

FIG. 8 diagrammatically shows an embodiment variant of the pressuredetector with a linear fiber portion, suitable for differential pressuremeasurement,

FIG. 9 diagrammatically shows a mode of forming optical gratings formeasuring stress and temperature variations, on two coupled opticalfiber portions,

FIG. 10 diagrammatically shows a variant of the mode of FIG. 9,

FIG. 11 diagrammatically shows a second mode of forming optical gratingson two portions of a single optical fiber,

FIG. 12 diagrammatically shows a mode of multipoint measurement of thethermal gradients within the detector body, and

FIG. 13 diagrammatically shows another mode of prestressing arectilinear fiber element.

DETAILED DESCRIPTION

The detector according to the invention is suited to measure pressurevariations in a fluid medium. It essentially comprises an outer rigidtubular housing 1 containing an element 2 deformable at least along theaxis of the housing under the action of pressure variations, a stresstransmission device 3 (described below) for applying a stress to atleast one optical fiber portion F1 provided with at least one opticalgrating B1 consisting of a Bragg grating, stressed between deformableelement 2 and another, fixed element isolated from the medium by rigidhousing 1, and an optical system for optically detecting thedeformations undergone by the optical grating as a result of thepressure variations undergone by the deformable element.

The body is preferably included in a sheath G made of a thermallyinsulating material so as to minimize the effects, on the measurements,of thermal gradients within the detector. This is useful in anyapplication where the temperature of the outside medium is low or thetime of exposure of the detector to a medium of relatively hightemperature is short.

Deformable element 2 comprises a base 4 fastened to housing 1 inside thelatter and a movable (mobile) part whose deformation is linked as it isknown in the art with the pressure variations to be measured. It can bea membrane or, as shown in the preferred example, a tubular bellows 5forming a continuation of base 4 towards the inside of housing 1 andended by a bottom 6. The axial displacement of bottom 6 of the bellowsis substantially proportional to the pressure variations. Its radialdisplacement is low. Its elongation variations are stable, whichprovides good reproducibility of the measurements in time as a functionof the pressure and of the temperature.

However, if the elongation variations are not stable, it is possible toadd additional temperature measurement points by means of Bragggratings, all along the detector body, whose number is predeterminedaccording to the desired measuring accuracy and to a known rule, so asto quantify the effect of the differential expansions induced by atemperature gradient and to correct the effects on the hydrostaticpressure measurement.

The shape and the dimensions of bellows 5, and the material from whichit is made, are of course suited for the axial displacement of thedeformable part to be compatible with the measurement possibilities ofthe Bragg grating B1 used and of the associated measuring device, andwith the pressure range to be measured. The walls of bellows 5 can be ofvariable thickness so as to limit the stresses towards the inside in thehollows of the corrugations without penalizing the amplitude of theaxial extensions. The bellows can be ring-shaped, saw-toothed,castellated, etc. The folds of bellows 5 can be axially symmetrical(forming rings) or helix-shaped to facilitate implementation.

A threaded passage 7 is provided through base 4, thus communicating theinside of bellows 5 with the medium. This communication can be direct orit can be provided by means of a more or less long tube T (fitting thethreaded opening) connecting the detector to the medium where pressure Pis measured.

Device 3 comprises a rigid tube 8 arranged inside housing 1. Tube 8 isfastened to base 4 at a first end and ended by a baseplate 9 at theopposite end. Bellows 5 is arranged inside rigid tube 8. Radial stops 10can be arranged between the part and inner tube 8 so as to limit itsradial deformation under the action of the outside pressure and toprevent frictions. In the case of a bellows, these radial stops 10 canbe rings made of two parts housed in the corrugations of the bellows.Inner tube 8 also comprises an axial safety stop 11 to limit axialextension of the bellows under the effect of the outside pressure.

I—Prestressed Fiber Loop Detector

The bottom or base 6 of the bellows comprises a semi-circular path 12through which an optical fiber 13 is passed. Path 12 can be delimited,for example, by the groove of a pulley portion 14 (a half pulley forexample) housed in a hollow provided in the wall of bottom 6 of bellows5. This pulley portion, whose axis is orthogonal to the axis of thebellows, is held in place on hollowed bottom 6 by fastening means thatare not shown but known to the man skilled in the art, such as cornerplates. It is also possible to substantially lengthen grooved bottom 6in relation to the non limitative representation of FIG. 1, so that itreaches the position of the axle of the pulley. In this case, stop 11has to be moved back accordingly. The pulley and its groove aredimensioned so as to induce as little friction as possible and to leaveit perfectly free in rotation in order to best limit the measuringerrors that might result therefrom. Pulley 14 is massive and rigid, andtightly secured to the wall of bottom 6 once set in place. Pulleyportion 14 has a diameter compatible with the optical signal attenuationdue to the curvature of the fiber, which is acceptable for this type ofmeasurement.

Device 3 also comprises a plate forming a stop 15 arranged on the sideof baseplate 9 opposite bellows 5. Baseplate 9 and stop plate 15 arerespectively provided with holes 16, 17 that are laterally offset andfacing one another, allowing passage of the strands of fiber 13 comingfrom either side of pulley 14. Fiber locking means 18 are used totightly secure the fiber at the level of holes 17.

These locking means 18 can comprise, for example, local oversizes S ofthe core of the fiber in form of a ball or sphere cooperating withtubular elements 19 of limited inside diameter that lodges itself inholes 17 of the stop plate. These locking means 18 can also comprise forexample mechanical elements such as ferrules (not shown) set on thefiber, that lock into holes 17 of stop plate 15.

It is also possible to stick the optical fiber in parts such as tubularelements 19 provided at the center thereof with a calibrated hole whosediameter is slightly larger than the outside diameter of the opticalfiber, so as to limit the measurement bias induced by the shearing understress of the glue and the fiber sheathing, this diameter of the holebeing sufficient to allow the glue to flow therethrough.

The measurement bias due to the sheath or glue shear, whatever thesolution selected for fastening the optical fiber, can be reduced byincreasing the length of the base of prestressed fiber F1 by asufficient length, this error being, at the first order, inverselyproportional to this length.

It is also possible to use a capstan type locking device by closing oneor more times the fiber loop coming from the semi-circular path (aroundpulley 14) by means of an opposite pulley nearby.

The stop plate comprises several bores for tension screws 20 restingagainst baseplate 9, allowing to move stop plate 15 away from baseplate9 and thus to prestress the fiber in its part F1 contained in rigidinner tube 8. It is in this part that the stressed optical fibercomprises at least one optical grating B1 (Bragg grating for example)whose deformations under the effect of the pressure variations to bemeasured are converted to measuring signals by optical system 25. One ormore lock screws 21 allow to lock stop plate 15 in relation to baseplate9 after prestressing fiber portion F1.

Base 6 is sufficiently thick to remain practically non-deformable whenthe bellows deforms. This base can be added (by welding for example) tothe end of the bellows. This added part can include pulley portion 14used to send the optical fiber towards stop plate 15.

In addition to the first Bragg grating B1 sensitive both to the stressvariations and to the temperature variations, the fiber preferablycomprises at least a second Bragg grating B2 in a portion F2 that is notsubjected to a stress, as described in connection with FIGS. 9-11. Thisallows to measure the local temperature variations and incidentallytheir gradient if the number of gratings is sufficient, and to eliminatethe biases of the pressure measurements obtained by means of opticalgrating(s) B1 under the effect of the temperature variations.

At its end opposite base 4, the cylindrical housing is connected to aconnecting tube 22. This tube is made of stainless steel for example.When the housing is externally exposed to the pressure to be measured,tube 22 is welded to the housing or connected thereto by apressure-tight connection. On the other hand, a simple connection issufficient if the detector is not directly exposed to the medium andreceives the pressure to be measured by means of a tube T connected tobase 4.

The two strands of fiber F entering and coming from tube 8 are connectedby optical connectors 23 of a well-known type or preferably welded totransmission fibers 24 running through the inside of this connectingtube 22 and are connected to an optical measuring device 25 of awell-known type suited for converting the deformations of fiber portionF1 to measurements of the medium pressure variations. The inner volumeof housing 1 is brought for example to a reference pressure which canbe, for example, the atmospheric pressure transmitted through the insideof connecting tube 22.

Rigid inner tube 8 is not in contact with the inner wall of housing 1likely to deform under the action of the outside pressure. Thus, thedistance variations between the bottom of deformable element 2 andinside tube 8 secured to base 4 of said element are not likely to beaffected by the pressure variations outside housing 1.

An embodiment using an inner tube 8 isolated against possibledeformations due to the effects of the outside pressure possibly exertedaround housing 1 has been described. Locking plate 15 could however restdirectly against the wall of the housing in the case where this wall iseither not exposed to pressure variations or sufficiently massive towithstand them without departing from the scope of the invention.

Consider for example the case of a standard Bragg grating fiber ofdiameter 125 μm, exclusive of sheathing. In the absence of any pressureapplied to the deformable element, the fiber is brought under permanentstress with a force of the order of 0.5 daN for example, within thelimit of the allowable elongation of the fiber which is of the order of0.5% for the planned working times, considering the aging of thestressed optical fiber. The shape and the dimensions of the bellows areso selected that, at the maximum pressure to be measured, the residualstress is reduced by half for example. Any increase in the pressure ofthe medium is translated into a stress decrease, a shortening of fiberportion F1 and correlatively a change in the deformation of opticalgrating B1, which the associated measuring system 25 is going tomeasure. During all the stages of exposure to pressure, the stressesexerted on the fiber are lower, which contributes to reducing theeffects of aging and therefore to increasing the life of the detector.

Zero adjustment of the detector is carried out for the maximum stressapplied. If the zero shifts, as a result of an initial prestressvariation, it can be readily corrected by acting upon plate 15.

By correlating in a well-known way the variations affecting grating B1,sensitive to both the pressure and the temperature, and grating B2sensitive to the temperature only, separate measurements of one and theother are obtained.

I-1 Differential Pressure Detector

The pressure detector that has been described can work as a differentialdetector by communicating the inside of the housing with a back pressurein relation to which the pressure variations of the medium are to bemeasured. This is possible only if the medium exerting the back pressureis not likely to deteriorate the stressed fiber, its prestress mechanism3 and communication fibers 24.

In the opposite case, two identical pressure detectors are used, eachwith a deformable element, a fiber prestressing assembly comprising atleast one or two optical gratings, separately connected to the samemeasuring device suited to combine the measurements of the two detectorsto deduce the pressure difference between their measurements.

According to the variant diagrammatically shown in FIG. 5, thedifferential pressure detector comprises, in a single housing 1, acentral compartment 26 in which a stable reference pressure P₀ prevailsand two lateral compartments 27, 28 communicating respectively with twomedia under respective pressures P1, P2. The two lateral compartments27, 28 are respectively separated from central compartment 26 by bellows30, 31 similar to the previous bellows 5. A device 3 (similar to theprevious device 3) allows to form and to stress, between bellows 30 and31, a open optical fiber loop F1 provided with at least two Bragggratings B1, B2 (in layouts such as those illustrated in FIGS. 9-11)from a fiber F entering central compartment 26, connected to an opticalmeasuring system similar to system 25. This embodiment is suitable forapplications where only measurement of the absolute value of pressuredifference |P₂−P₁| is sought.

In the variant diagrammatically shown in FIG. 6, two identical pressuredetectors are used, possibly in a single housing 1, each with adeformable element such as a bellows 30, 31, a device 3A, 3B forprestressing an optical fiber portion F1 forming an open loop andcomprising at least one or preferably at least two optical gratings B1,B2 which are separately connected to a single measuring device 25 suitedto combine the measurements of the detectors in order to deduce thepositive or negative pressure difference between their respectivemeasurements.

II Prestressed Rectilinear Fiber Portion Pressure Detector

In the second embodiment of FIG. 7, the same reference numbers designatethe same elements as in FIG. 1. It differs from the first oneessentially in that fiber portion F1 forming a loop is here replaced bya prestressed rectilinear fiber portion F′1. At a first end, it is keptlocked in a first mechanical latching means 32 of a well-known typefastened to bottom 6 of tubular bellows 5. At the opposite end, fiberportion F′1 is kept locked in a second latching element 33 fastened tomobile stop plate 15. Fiber prestress is obtained by moving the stopplate away from fixed baseplate 9 of inner tube 8. Similarly, a firstBragg grating B1 is formed on fiber portion F′1, and a second grating B2is formed on another non-stressed portion of the fiber (see FIG. 9-11).It can be noted that the number of gratings can be increased to increasethe measuring accuracy, whether pressure or temperature measurements,and to calculate the gradients.

In order to maintain fiber portion F1 stressed, it is possible to use aslatching element 32 parts such as tubular elements 19 (see FIG. 1)provided at the center thereof with a calibrated hole of diameterslightly larger than the outside diameter of the optical fiber, and toimmobilize the ends of the fiber portion by sticking, the diameter ofthe hole being sufficient to allow the glue to flow therethrough. Themeasurement bias induced by the shearing under stress of the glue and ofthe fiber sheathing is thus limited.

Similarly, the increase, by a sufficient length according to a knownrule, of the base length of prestressed optical fiber F1 allows thesemeasurement biases to be reduced.

According to the embodiment of FIG. 13, tube 8 is here open at its endopposite that resting on base 4 (FIG. 1). A first element Al forlatching fiber portion F′1 is similarly secured to bottom 6 of bellows5. The opposite fiber portion latching element A2 is secured to acylindrical part 34 comprising a head 35 whose diameter is substantiallyequal to the inside diameter of tube 8, and a cylindrical extension 36threaded over part of its length, itself continued by a terminal part37. An axial canal 38 runs right through part 34, allowing passage ofthe fiber towards the outside. When part 34 is engaged in tube 8, tube 8is closed behind it by means of an annular plate 39 that is fastened totube 8 by radial fastening screws 40. Belleville type washers areinterposed between head 35 and plate 34. A nut 42 screwed on threadedpart 36 of part 34 allows, by moving the latter back, to exert anadjustable prestress on fiber portion F′1. During tightening, terminalpart 37 is held in place so that the prestressed fiber portion undergoesno torsion. When the prestress is set at the predetermined value, radialscrews 43 allow cylindrical part 34 to be immobilized in relation totube 8. Nut 42 can then be immobilized by means of a counternut (notshown).

The Bragg grating B2 allowing temperature compensation of the lengthvariations of fiber portion F′1 measured by Bragg grating B1 can beplaced on a fiber portion connected (by a connector C) or welded inparallel to the non-prestressed fiber F′2. It is also possible toconnect this grating B2 in series with grating B1 on a non-prestressedportion of the fiber, preferably as close as possible thereto.

The length of optical fiber F′1 between latching elements A1, A2 can bereadily changed by changing the length of tube 8. This allows tominimize the influence of a possible sliding of prestressed fiberportion F′1 inside the latching elements and to increase the resolutionof the detector.

II-1 Differential Pressure Detector

In the variant shown in FIG. 8, two identical pressure detectors areused, possibly in a single housing 1, each with a deformable elementsuch as a bellows 30, 31, a device 3A, 3B for prestressing a rectilinearoptical fiber portion F1, each prestressed between two latching elements32, 33, which comprises at least one or preferably at least two opticalgratings B1, B2, and are separately connected to a single measuringdevice 25 suited to combine the measurements of the two detectors so asto deduce therefrom the positive or negative pressure difference betweentheir respective measurements.

III Detectors Setup

The second Bragg grating B2 must imperatively be formed on an opticalfiber portion free from any stress so as to detect only the variationslinked with the temperature variations. A setup such as thosediagrammatically illustrated in FIGS. 9, 10 is used for example, wheresecond grating B2 is formed in the vicinity of the end of a fiberportion F3 connected to fiber portion F1 (stressed between the twopoints A1, A2) by an optical coupler C of a well-known type. The fiberportion where grating B2 is formed can be freely installed in amicrotube (FIG. 9) or stuck, prestressed, in a metal tube (FIG. 10). Ifthere is enough room for installing Bragg grating B2 on the same fiberas grating B1, the embodiment of FIG. 11 can be selected. Grating B2 ishere on the end of fiber portion F1 outside the fiber portion stressedbetween the two points A1, A2. This embodiment saves using an opticalcoupler C which causes considerable optical losses.

IV Measuring Accuracy Improvements

IV-1 Detector Sensitivity

In order to improve the sensitivity of the detector, it is possible tomultiply the number of gratings and the number of optical fibers, theresolution being improved as 1/√{square root over (n)} where n is thenumber of gratings used to provide the measurement.

In order to limit the measurement biases introduced by the imperfectionsof the latching points of prestressed fiber F1, F′1, it is advisable tobest increase its length, the measurement biases being, at the firstorder, inversely proportional to this length.

IV-2 Correction of the Measurement Biases Due to Thermal Stresses

Body 1, rigid inner tube 8, base 4 serving as fixed reference for theelongation measurements and bellows 5 fastened thereto do not have thesame length and therefore have different expansions, which may have theeffect of applying parasitic differential stresses to prestressed fiberportion F1. They can be minimized by judiciously selecting the materialsused for manufacturing them respectively, so that the distance betweenbase 4 and bottom 6 of bellows 5 only depends on the pressure variationsexerted on the bellows.

D and L being the unequal distances in relation to base 4 (FIG. 1)respectively of latching points A1 and A2, it is advisable to select theexpansion coefficients k₁, k₂ of the metals from which bellows 5 on theone hand and tube 8 on the other are respectively made in such a waythat $\frac{L}{D} \approx {\frac{k_{2}}{k_{1}}.}$

The constituent materials of the various parts of the detector aregenerally selected according to their thermal conductivity coefficient cso as to reduce the thermal exchanges with the outside medium and toprevent formation of thermal gradients within the detector. Materialshaving a high thermal conductivity are thus selected for the partsinside the detector, and materials having a lower thermal conductivityare selected for the parts in thermal contact with the outside medium.

If the compensation obtained is considered to be insufficient,considering the high measuring accuracy expected, additionalcompensations can be introduced by means of systematic measurements ofthe specific thermal expansion of each part inside the detector, usingfor example other Bragg gratings.

In the simplified diagram of FIG. 12, a non-stressed optical fiberportion F3 along which several gratings BT1, BT2, . . . , BTn allowingto precisely measure the thermal variations of the different parts areinscribed is interposed between each bellows 5 and body 1 orintermediate rigid tube 8 (FIG. 1), this fiber portion being connectedto stressed fiber portion F1 by an optical coupler C. Optical system 25is suited to combine the measurements of the various gratings B2 inorder to generate the fine correction to be applied to the pressuremeasurements produced by each grating B1.

V Multipoint Pressure Measurements

Several similar pressure detectors can be installed in series ontransmission fibers 24 connected to measuring system 25. Each one,because of the specific pitch of the Bragg gratings B1, B2 engravedthereon, has an individual feature which allows to discriminate bymultiplexing its own contribution in the light spectrum reflected by thevarious detectors to measuring device 25.

VI Isolation of the Bellows in Relation to the Measurement Medium

The medium in which the detector is dipped may be corrosive (chemical orelectrochemical corrosion for example) to the point where thecharacteristics of the deformable element (membrane, bellows) change,which might distort the measurements. It is possible, in this case, toisolate the bellows from the outside fluid by using an intermediatefluid providing transmission of the pressure. This intermediate fluid isisolated from the corrosive outside fluid by a deformable membrane orbellows consisting of a material withstanding the corrosive fluid andwhich is in equipressure with the corrosive fluid for which the pressureis measured.

1. An optical fiber pressure detector for detecting pressure variationsin at least one medium in relation to a reference medium, comprising atleast one deformable element inside a body, exposed on one side to thereference medium and, on the opposite side, to a pressure to bemeasured, at least one optical fiber at least a portion of which isconnected on one side to deformable element and, on the opposite side,to a reference point, and whose stress varies with the displacements ofthe deformable element, this optical fiber portion comprising at leastone optical grating, an optical system for detecting the deformationsundergone by each optical grating as a result of the pressure variationsundergone by the deformable element and means for adjusting the stressapplied to the optical fiber, characterized in that the deformableelement comprises a bellows provided with a base fastened to body, thereference point being formed by means of a device comprising a rigidtube provided with a bottom, resting at its end opposite the bottomagainst base of the bellows, a rigid element translatable in thedirection of elongation of the optical fiber, which rests against bottomof rigid tube by means of adjustable spacing means, and retention meansallowing the optical fiber portion to be fastened to rigid element.
 2. Apressure detector as claimed in claim 1, wherein device comprises meansfor giving each optical fiber portion the shape of an open loop formedbetween bellows and said retention means.
 3. A pressure detector asclaimed in claim 2, wherein the semi-circular path consists of thegroove of a pulley portion whose axle is fastened to deformable element.4. A pressure detector as claimed in claim 1, comprising means forfastening the optical fiber portion respectively to the end of bellowsand to the rigid element.
 5. A pressure detector as claimed in claim 1,wherein the means for prestressing fiber portion comprise a stop platekept away from bottom of rigid tube by adjusting screws resting againstbottom, the optical fiber portion retention means being associated withthis stop plate.
 6. A pressure detector as claimed in claim 5, whereinthe retention means comprise at least one locking means embedded in stopplate which cooperates with at least one local oversize of optical fiberportion.
 7. pressure detector as claimed in claim 5, wherein theretention means comprise at least one piece set in fiber portion.
 8. Apressure detector as claimed in claim 5, wherein the retention meanscomprise two mechanical latching elements suited to hold each fiberportion locked respectively at its opposite ends, one fastened todeformable element, the other to stop plate.
 9. A pressure detector asclaimed in claim 7, wherein the retention means comprise at least onelocking part embedded in stop plate pierced with a calibrated hole, eachfiber portion being associated by sticking with each locking part.
 10. Apressure detector as claimed in claim 5, wherein the means forprestressing each fiber portion comprise means for locking stop plate inrelation to bottom of rigid tube.
 11. A pressure detector as claimed inclaim 4, wherein the retention means comprise two mechanical latchingmeans suited to hold each fiber portion locked respectively at itsopposite ends, one fastened to the end of bellows, the other to a parttranslatable in relation to bottom of rigid tube, and means foradjusting to a predetermined value the prestress applied to fiberportion.
 12. A pressure detector as claimed in claim 11, wherein partcomprises a head and an extensions, the adjustment means comprise anadded annular plate forming the bottom of rigid tube which is providedwith a central opening for passage of extension, and means allowingtranslation of extension by screwing, resting against said annularplate.
 13. A pressure detector as claimed in claim 1, comprising opticalfiber elements connected to optical system.
 14. A pressure detector asclaimed in claim 1, wherein the bellows is arranged in body in such away that its length increases when the pressure in the medium increases.15. A pressure detector as claimed in claim 14, comprising protectionmeans for limiting deformations of bellows.
 16. A pressure detector asclaimed in claim 14, wherein the bellows comprises symmetrical orasymmetrical corrugations.
 17. A pressure detector as claimed in claim14, wherein the corrugations of the bellows are helical, withcorrugation amplitudes of the inner and outer helices that can be equalor different.
 18. A pressure detector as claimed in claim 1, comprisingan outer sheath made of a thermally insulating material.
 19. A pressuredetector as claimed in claim 1, wherein body is made of a low thermalconductivity material and the elements of the detector inside body aremade from materials whose thermal conductivities are so selected as tobest minimize the formation of thermal gradients.
 20. A pressuredetector as claimed in claim 1, comprising at least a second opticalgrating on another fiber portion that is not subjected to a prestress.21. A pressure detector as claimed in claim 1, comprising a plurality ofoptical gratings distributed over at least one fiber portion that is notsubjected to a prestress, for detecting thermal gradients within body,the optical system being suited to combine the measurements of thevarious gratings so as to best correct the effects of the temperature onthe pressure measurements.
 22. A pressure detector as claimed in claim1, wherein the reference medium is at atmospheric pressure or a backpressure.
 23. A pressure detector as claimed in claim 1, comprising twodeformable elements exposed on one side to the reference medium and, onthe opposite side, respectively to two pressures to be measured, atleast one optical fiber portion subjected to a prestress, which isconnected on one side to one of the deformable elements and, on theopposite side, to a rigid part of the other deformable element formingsaid second rigid element, whose stress varies with the displacements ofthe two deformable elements, this prestressed optical fiber portioncomprising at least one optical grating.
 24. A pressure detector asclaimed in claim 1, comprising two deformable elements exposed on oneside to the reference medium and, on the opposite side, respectively totwo pressures to be measured, at least two optical fiber portions eachsubjected to a prior prestress, which are connected on one siderespectively to the two deformable elements and, on the opposite side,to at least one reference point which is fixed, and whose respectivestresses vary separately with the displacements of the two deformableelements, each one of these prestressed optical fiber portionscomprising at least one optical grating.
 25. A pressure detector asclaimed in claim 1, comprising on each optical fiber a plurality ofgratings so as to increase the resolution of the detector.
 26. Apressure detector as claimed in claim 1, wherein the base length of theprestressed optical fiber portion is sufficiently great to limit themeasurement biases due to imperfections in the latching system.
 27. Apressure detector as claimed in claim 1, wherein optical fibers providedwith standard or phase jump type Bragg gratings are used.
 28. A pressuredetector as claimed in claim 1, wherein the second rigid elementcomprises a tube isolated from the medium by outer housing.
 29. Apressure detector as claimed in claim 1, wherein the thermal expansioncoefficients of the constituent elements of the detector are selected soas to best minimize the length variations of prestressed fiber portionunder the effect of the temperature variations.